Ion source for positive ion accelerators



IIIIIII/IIIIIIIIII/4 "qI/IM/I/mIl/IIIIIIIII/III] Feb. 18, 1964 N. B. BROOKS ETAL ION SOURCE FOR POSITIVE ION ACCELERATORS Filed Sept. 22 1960 P/Q/OE APT United States Patent ION SOURCE FOR POSITIVE ION ACCELERATORS Norman B. Brooks, Bedford, and Bertram S. Quintal,

Peabody, Mass., assignors to High Voltage Engineering Corporation, Burlington, Mass, a corporation of Massachusetts Filed Sept. 22, 1960, Ser. No. 57,326 2 Claims. (Cl. 31363) This invention relates to duoplasmatron-type ion sources and in particular to a novel arrangement of the electron-emitting filament of such an ion source.

In the conventional duoplasmatron-type ion source the electron-emitting filament is in axial alignment with the apertures in the magnetic grid, in the anode, and in the extraction electrode. As a result, high energy electrons come back up through the apertures and strike the filament, thus damaging or destroying it. These electrons originate in the vicinity of the extractor electrode and hence are at a high negative potential.

In accordance with the invention the electron-emitting filament is removed from the axis of the ion source and in the place previously occupied by th filament in prior art devices a robust type of emitter is placed, such as a sintered cathode or any electron emitting material such as tantalum, a rare earth oxide, tungsten oxide, etc.

The invention may best be understood from the following detailed description thereof, having reference to the accompanying drawing in which:

FIG. 1 is a diagrammatic view of a conventional duoplasmatron-type ion source; and

FIG. 2 is a diagrammatic view of an ion source embodying the invention.

The conventional duoplasmatron ion source is more fully described in the following material: Manfred Von Ardenne, Tabellen der Elektronenphysik Ionenphysik und Ubermikroskopie (Deutscher Verlag der Wissenschaften, Berlin, 1956), page 544; The Review of Scientific Instruments, volume 30, pages 694 to 699, August 1959; Proceedings of the IRE, volume 40, page 645, June 1952; British Patent 680,347; British Patent 680,350.

Referring to FIG. 1 of the drawing, in the conventional duoplasmatron type ion source there are two concentrated regions of plasma, shown in FIG. 1 at 1 and 2. One such region, shown at 1 in FIG. 1, is produced by providing a restricted aperture in the vicinity of the anode between the cathode and the anode and the other of which is produced by means of an inhomogeneous magnetic field of a pole face lens. Electrons produced at a cathode 3 are accelerated towards an anode 4 by the electric field which is produced by impressing a potential difference in the order of 10 volts, in the present example of, say, 500 volts therebetween by means of a voltage source 5. The anode 4 is provided with an emission aperture 6 through which ions are emitted. A first magnetic pole face 7 is provided between the cathode 3 and the anode 4 in the vicinity of the anode 4 and acts as an intermediate electrode. This magnet pole face 7 has a capillary 8 through it which may have a diameter, for example, of about millimeters. The small aperture of this intermediate electrode 7, which is maintained at about 70 to 180 volts by the potential divider 9, serves to confine the discharge, and in the vicinity of the anode 4, the plasma takes the form of a bubble 1 which is bounded by an electrical double layer. The length of the capillary 8 must be so measured that the discharge mechanism in the region 1 of the double layer bubble is not disturbed by the fringing magnetic field of the pole face lens 7, which is main-. tained at about 70 to 180 volts by the potential divider 9. In this double layer the electrons of the thinner plasma on the side of the cathode 3 are accelerated and in the geometry shown are focused exactly on the region in front of the capillary 8. As a result, a particularly thick plasma is produced in the region 1 in front of the capillary 8 in the intermediate electrode 7. This effect is that of the unoplasmatron which is described in the book by Von Ardenne referred to above at page 543 and also in British Patent No. 795,766.

Additional improvement is provided by the inhomogeneous field of the pole face lens comprised by the magnetic pole face 7 and the anode 4 which acts as a second magnetic pole face, a tungsten insert 10 being provided at the center of the anode 4 because of ion bombardment. The effect of the inhomogeneous magnetic field is described on pages 124 and 125 of the book by Von Ardenne referred to above, and the effect has been described as the production of a magnetic mirror field which acts to reflux the electrons so that escape is possible only very near to the axis. An extraction electrode 11 is maintained at a negative potential in excess of 10 volts, and in the present example preferably at a potential of -50 to 70 kilovolts; the extracting field does not operate on the plasma itself, but only on those ions which have escaped from the plasma through the anode emission aperture 6.

Referring now to FIG. 2, the discharge is initiated by electrons emitted from the filament 12 which, in accordance with the invention, is removed from the axis of the ion source. These electrons are attracted towards the magnetic grid 13 and those which arrive in the central region of the magnetic grid will see the potential on the anode 14 which has a very small emission aperture 15. This causes at least some of the electrons to be deflected and head down towards the capillary 16 in the magnetic grid 13. To insure that this does occur a deflecting plate 17 may be provided opposite the filament 12 at the po tential of the filament 12 so as to deflect the electrons down towards the capillary 16 in the magnetic grid 13. Upon reaching the capillary 16 in the magnetic grid 13, as in theconventional duoplasmatron source, electrons are caught in a magnetic mirror and produce ionization in the gas in the ion source. Positive ions having been formed in this plasma, an extractor electrode 18 operates to draw many of them out and with the source now initially in operation, the high energy electrons which are accelerated back along the ion source axis, formed by the capillary 16 in the magnetic grid 13 and the emis sion aperture 15 in the anode 14, strike a sintered cathode 19, or other robust electron emitter placed on the ion source axis, so as to heat the same to electron emitting temperatures. This sintered cathode 19 then acts as the principal source of electrons in the operation of the source so that the only bombardment of the filament 12 is that occasioned by positive ions.

Having thus described the principles of the invention, together with an illustrative embodiment thereof, it is to be understood that although specific terms are employed, they are used in a generic and descriptive sense, and not for purposes of limitation, the scope of the invention being set forth in the following claims.

We claim:

1. An ion source comprising in combination a cathode adapted to emit electrons upon electron bombardment thereof, an apertured magnetic pole-face lens, and an apertured extraction electrode in axial alignment, said cathode and the apertures of said lens and said extraction electrode defining an axis, said lens comprising an apertured anode and an apertured grid, means for impressing a potential not greater than of the order of 10 volts negative upon said cathode with respect to said anode, means for impressing a potential not less than 10 volts negative upon said extraction electrode with respect to said anode,

'13 a filamentary thermionic electron emitter supported away from said axis but sufliciently near it so that electrons emitted thereby come Within the influence of the potential difference between said cathode and said anode.

2. An ion source comprising in combination a cathode adapted to emit electrons upon electron bombardment thereof, an apertured magnetic pole-face lens, and an apertured extraction electrode in axial alignment, said cathode and the apertures of said lens and said extraction electrode defining an axis, said lens comprising an apertured anode and an apertured grid, means for impressing a potential not greater than of the order of 10 volts negative upon said cathode with respect to said anode, means for impressing a potential not less than 10 volts References Cited in the file of this patent UNITED STATES PATENTS 2,668,260 Barnett et a1. Feb, 2, 1954 2,772,362 Dietz Nov. 27, 1956 2,978,580 Von Ardenne Apr. 4, 1961 

1. AN ION SOURCE COMPRISING IN COMBINATION A CATHODE ADAPTED TO EMIT ELECTRONS UPON ELECTRON BOMBARDMENT THEREOF, AN APERTURED MAGNETIC POLE-FACE LENS, AND AN APERTURED EXTRACTION ELECTRODE IN AXIAL ALIGNMENT, SAID CATHODE AND THE APERTURES OF SAID LENS AND SAID EXTRACTION ELECTRODE DEFINING AN AXIS, SAID LENS COMPRISING AN APERTURED ANODE AND AN APERTURED GRID, MEANS FOR IMPRESSING A POTENTIAL NOT GREATER THAN OF THE ORDER OF 10**2 VOLTS NEGATIVE UPON SAID CATHODE WITH RESPECT TO SAID ANODE, MEANS FOR IMPRESSING A POTENTIAL NOT LESS THAN 10**4 VOLTS NEGATIVE UPON SAID EXTRACTION ELECTRODE WITH RESPECT TO SAID ANODE, A FILAMENTARY THERMIONIC ELECTRON EMITTER SUPPORTED AWAY FROM SAID AXIS BUT SUFFICIENTLY NEAR IT SO THAT ELECTRONS EMITTED THEREBY COME WITHIN THE INFLUENCE OF THE POTENTIAL DIFFERENCE BETWEEN SAID CATHODE AND SAID ANODE. 